The basic concept of a multi-roll shape-correction leveler (hereinafter shape-correction leveler or just leveler for brevity) has been known for many years. Shape-correction levelers were developed to account for the deficiencies of known hot rolling mills and the undesirable shape defects hot rolling mills commonly impart to the metal strip produced thereby. Common forms of such shape defects are shown in FIGS. 1A-1D to include coil set, cross bow, edge wave, and center buckle, respectively.
Shape-correction levelers typically use opposing, substantially parallel sets of multiple work rolls that often are supported by back-up rolls and associated bearings designed to withstand high separating forces and to control the bending and deflection of the work rolls. The work rolls are normally positioned so that an upper row of work rolls are located above a cooperating lower row of work rolls. A gap of adjustable dimension is normally present between the upper and lower work rolls.
During a flattening operation, metal strip (typically from a coil) material is fed into the entrance of the leveler whereafter it is caused to pass between the opposing sets of work rolls 5, 10 (see FIG. 2). Each set of work rolls is placed into contact with the metal strip by driving one set of work rolls toward the other so that a leveling (flattening) force is impressed upon the metal strip as it passes therebetween. More specifically, contact between the work rolls and the metal strip material causes the metal strip to be repeatedly bent up and down (i.e., to S-wrap through the work rolls) as it passes through the work rolls, which repeated bending of the metal strip material removes stresses induced therein by the hot rolling process. Such a shape-correction leveler may be used to impart flatness across the entire width of a metal strip.
A shape-correction leveler may also be operated to selectively apply forces of different magnitudes to different areas of a strip of material passing therethrough. This selective application of force allows particular zones of the strip of material (from edge to edge) to be worked more than other zones as the strip passes through the leveler. Thus, shorter zones of the strip may be selectively elongated to match the length of the longer zones. This allows a shape-correction leveler to correct a variety of different shape defects. A typical shape-correction leveler setup 15 for correcting center buckle is shown in FIG. 3A, while a typical setup 20 for correcting edge wave is shown in FIG. 3B.
Each work roll of a typical shape-correction leveler is normal driven to propel the strip of material through the leveler during a leveling (flattening) operation. A shape-correction leveler drive system commonly consists of a main motor, a reduction gearbox, and a pinion gearbox, that cooperate to provide output rotation to each work roll.
An interesting phenomenon occurs when the work rolls of a shape-correction leveler penetrate into a strip of material being processed and the material S-wraps through the work rolls. With light penetration (e.g., at the exit end of the leveler) the roll surface speed substantially matches the strip speed. However, when the rolls penetrate deeper (e.g., at the entry end of the leveler), the roll surface speed tends to run slower than the strip speed. This phenomenon occurs because the material has a bend radius, (entry end of leveler) the surface speed of the material on the inside of the bend radius is moving slower than the surface speed on the outside of the bend radius (see FIG. 4). For purposes of illustration, one helpful analogy would be wheel speed on an automobile, wherein the wheels on both sides of the automobile rotate at the same RPM when the automobile is going straight, but the wheels on the inside of the curve will rotate slower than the wheels on the outside of the curve when the automobile is making a turn. In the case of a shape-correction leveler, the work rolls are contacting the inside radius of the bending strip material, so the rolls on the entry end of the leveler want to run slower to match this slower inside radius surface speed. One example of differential roll speed from an entry to an exit end of an exemplary leveler is depicted in FIG. 5.
This phenomenon may be referred to as differential roll speed (DRS). When the leveler work rolls are all driven together at the same speed (see e.g., FIG. 4 and FIG. 6), the entry rolls try to push the strip material through the exit rolls, while the exit rolls try to hold the material back. This DRS causes several issues in the leveler. One issue is that when the work rolls are geared together, the DRS causes high loading on the entry work rolls and internal torque windup within the roll drive system—which may cause premature failure of the drive components. Another issue is that more power consumed tends to be consumed when the work rolls are fighting each other. Yet another issue is that DRS tends to cause a compression of the strip material in a leveler rather than a stretching of the material, which reduces the effectiveness of the leveler.